Original post by Martin Gruau, Consumer Operations (from official gmail blog)
To install any of these gadgets, follow those steps:
1) From your Gmail account, go to the Labs tab of Gmail Settings.
2) Look for the Lab "Add any gadget by URL." Enable it, then click "Save changes."
3) Go to the new "Gadgets" tab under "Settings" and add the relevant .xml address.
Look for a specific query right from Gmail.
Make some quick calculations while typing an email.
Add a sticky note to the corner of your Gmail account.
Remember the Milk
If you're a fan of this task management system, accessing all your "Remember the milk" notes from right within Gmail can be super handy.
Gives you the time of day for any place in the world.
A real time currency converter.
bit.ly URL shortener
Lets you shorten URLs in a single click.
Chuck Norris fact generator
Displays a different "Chuck Norris fact" every day of the year.
Finally, you might be familiar with the last two gadgets, since they are also available as individual Gmail Labs:
Displays your Google Calendar agenda right from Gmail.
Gives you quick access to your most recent documents.
You can find lots of other gadgets optimized for iGoogle on this page, and many of them work well in Gmail, too.